Optical Fibers have become the communication medium of choice for long distance communication due to their excellent light transmission characteristics over long distances and the ability to fabricate such fibers in lengths of many kilometers. The transmitted light can also power the sensors, thus obviating the need for lengthy electrical wires. This is particularly important in the petroleum and gas industry, where strings of electronic sensors are used in wells to monitor downhole conditions.
As a result, in the petroleum and gas industry, passive fiber optic sensors are used to obtain various downhole measurements, such as, pressure or temperature. A string of optical fibers within a fiber optic system is used to communicate information from wells being drilled, as well as from completed wells. For example, a series of weakly reflecting fiber Bragg gratings (FBGs) may be written into a length of optical fiber, such as by photoetching. As is known in the art, the distribution of light wavelengths reflected from an FBG is influenced by the temperature and strain of the device to which the FBG is attached. An optical signal is sent down the fiber, which is reflected back to a receiver and analyzed to characterize the length of optical fiber. Using this information, downhole measurements may be obtained.
Due to the depth of typical oil and gas wells, one or more of the optical sensors are typically spliced into a length of optical transmission fiber that extends from the surface to the desired depths. As such, it is desirable to take measurements at various depths of the well. Additionally, while drilling optical sensors can be added to the monitoring system for adequate coverage of the well.
For some applications, it is important to determine the precise location of a tool or the open/closed status of a valve with optical sensors, which heretofore has not been disclosed in the art.